As progress in higher performance of a computer is made, further reduction in size, and increase in capacity are being required of the hard disk drive that is the main storage unit thereof. To cope with such requirements, it is desirable to enhance recording density. With conventional longitudinal magnetic recording, however, a recording medium has portions where the recording bits adjacent to each other have opposite magnetizations for carrying written information, so that if a bit length is shortened, a magnetization state is turned unstable, causing a problem of thermal stability, that is, difficulty with holding information for a long time. There is a perpendicular magnetic recording as one of means for circumventing this problem. With perpendicular magnetic recording, magnetization for carrying information is oriented in the direction of thickness of a recording medium, so that a magnetic field generated from a recording bit acts in such a direction as to stabilize magnetization in an adjacent recording bit, and consequently, the problem of the thermal instability hardly occurs.
One problem with a write head for perpendicular magnetic recording is a possibility that when an external field acts thereon, magnetic material disposed inside the head will behave like an antenna to cause the external field to converge thereto, thereby writing unnecessary information to a recording medium, and resulting in erasure of necessary information. In order to reduce such an adverse effect of the external field, and to enhance the external field durability, there is the need for improvement on not only a main pole and a return pole as constituents of a write head, but also shield layers as constituents of a read head.
For the head for the perpendicular magnetic recording to achieve a still higher recording density, a head of a CPP (current perpendicular to the plane) structure where a sensing current is caused to flow so as to penetrate through multi-layer interfaces of a sensor film is preferably used as a read head. That is, in comparison with a conventional head of a CIP (current into the plane) structure where a sensing current is caused to flow in the in-plane direction of a sensor film, the head of the CPP structure has following advantages over the conventional head of the CIP structure.
(1) As there is no need for securing insulation between the sensor film and a lower shield or an upper shield, it is possible to dispense with an insulating layer that is indispensable for the head of the CIP structure, so that a distance between the lower shield and the upper shield (a shield-to-shield distance) can be rendered narrower.
(2) With the head of the CIP structure, the sensor film is provided with a thick electrode on respective sides thereof so as to allow the sensing current to flow in the in-plane direction of the sensor film, thereby causing a shield-to-shield distance on both sides of the sensor film to become wider. As a result, a shielding effect in a cross-track width direction is lessened, thereby leading to occurrence of a problem of side reading, causing signals in wider range to be read. In contrast, with the head of the CPP structure, since electrodes on both sides of the sensor film is unnecessary, a narrow shield-to-shield distance can be maintained even on both sides of the sensor film, and as a result, it is possible to achieve a narrower magnetic track width.
However, there is a demerit due to adoption of the CPP structure for the head. The demerit is related to description given in item (1) as above. More specifically, because above and below the sensor film of the head of the CIP structure, there is disposed an alumina insulating layer higher in hardness, and smaller in a coefficient of thermal expansion in comparison with a common metal, even in case that any distortion occurs to the shield layers that are large in volume within a read head, effects of the distortion on the sensor film are relieved by the alumina insulating layer. On the other hand, since the sensor film of the head of the CPP structure is in direct contact with the shield layers or in contact therewith through a metal layer provided for adjustment of the shield-to-shield distance, the sensor film is susceptible to the effects of distortion occurring to the shield layers. In JP-A No. 2004-334995, there has been disclosed a technology for using a shield small in thermal distortion, in other words, small in thermal expansion, for a shield layer of a head of the CPP structure.
In order to cope with requirements for a higher recording density, it is essential to combine the write head for perpendicular magnetic recording with the read head of the CPP structure. In JP-A No. 2004-334995, use of a low thermal expansion non-magnetic material having a coefficient of thermal expansion not greater than 11.5×10−6 (/° K) for at least either of a lower magnetic shield layer and an upper magnetic shield layer has been disclosed as a technology for curbing one of problems with the head of the CPP structure, that is, the effects of the distortion occurring to the shield layers. In this case, however, a countermeasure against the external field durability, posing a problem when the read head of the CPP structure is combined with the write head for perpendicular magnetic recording, has not been taken into consideration.